The incorporation of metallic films embedded in a semiconductor device, thereby creating a hybrid structure, has received significant attention. This has primarily been motivated for two reasons: (i) because of the low resistivity of the metals that would enable fabrication of buried contacts, interconnects and ground planes, which are considered essential to the three dimensional integration of semiconductor devices; and (ii) because of the possibility of combining the transport properties of metals and semiconductors to create novel or improved device applications, for example metal base transistors or high frequency switches.
Synthesis of metal/semiconductor hybrid structures impose stringent requirements on the film and heterojunction properties. Specifically, the crystalline quality and thermal stability of both the embedded metallic layer and the semiconductor overgrowth, as well as the integrity of the heterointerfaces, are of paramount importance. While the epitaxial growth of metals on semiconductors is now relatively routine with low temperature growth techniques such as molecular beam epitaxy, for example epitaxial metallization systems reported to be grown on GaAs are Al, Ag, Fe, and body-centered-cubic Co among others, see A. Y. Cho and P. D. Dernier, J. Appl. Phys., 49.3328 (1978); J. Massies, P. Delescluse, P. Etienne and N. T. Linh, Thin Solid Films, 90, 113 (1980); J. R. Waldrop and R. W. Grant, Appl. Phys. Lett., 34, 630 (1979); G. A. Prinz and J. J. Krebs, Appl. Phys. Lett., 39.397 (1981); and G. A. Prinz, Phys. Rev. Lett., 54, 1051 (1985) which are incorporated herein by reference, it is noted that these metal/semiconductors heterostructures are not thermodynamically stable since the metals will react with As and/or Ga to form metal-As (M-As) and metal-Ga (M-Ga) compounds. In addition, the Al-containing compounds are susceptible to an Al/Ga exchange reaction at elevated temperatures. A much more serious limitation has been the inability to grow high quality single crystal semiconductor films on deposited metal layers.
Key factors for consideration in fabricating hybrid structures have involved the differences in crystal structure, thermal stability, bonding disparities, and growth compatibility of the constituent materials. While such factors have seriously hindered the fabrication of buried metal structures, the growth of a semiconductor on metal (NiAl/GaAs) has been reported. See T. Sands, Appl. Phys. Lett., 52, 197 (1988), and J. P. Harbison, T. Sands, N. Tabatabaie, W. K. Chan, L. T. Florez, and V. G. Keramidas, Appl. Phys. Lett., 53, 1717 (1988) which are incorporated herein by reference. However, fabrication of only a single semiconductor/metal layer has been achieved. An alternative approach has involved incorporating a semimetal, rather than a conventional metal, into the heterostructure. Recently, ErAs has been successfully grown on GaAs and shown to exhibit semimetallic behavior. See C. J. Palmstrom, N. Tabatabaie, and S. J. Allen, Jr., Appl. Phys. Lett., 53, 2608 (1988), which is incorporated herein by reference. Unfortunately, the resistivity of this structure was measured to be 7.2 m.OMEGA.cm--nearly four orders of magnitude higher than the resistivity of copper, and like the prior art NiAl/GaAs structure, only a single layer has been successfully incorporated into a heterostructure. In addition, these prior art systems have inherent problems associated with materials incompatibilities, and differing crystal structures that will continue to plague the performance of electronic devices fabricated with these structures. Put simply, none of the prior art metal/semiconductor or semimetal/semiconductor structures that have been fabricated have achieved optimal heterostructure properties required of potential applications for these structures.
The present invention enables the fabrication of stable, high quality semimetal/semiconductor interfaces and multiple semimetal/semiconductor layers. In addition, the present invention enables semimetal layers in a semimetal/semiconductor multilayer to be fabricated such that the semimetal becomes a semiconductor. The semimetal/semiconductor heterostructures of the present invention are believed to have applications in microelectronics devices, especially high speed microelectronics, optical devices, mesoscopic physics, and potentially high-temperature superconductivity.